1. Field of the Invention
The invention is based on a limit current sensor for determining the lambda value in gas mixtures of the generic type of the main claim.
2. Description of the Related Art
The German Offenlegungsschrift 39 08 393 discloses a limit current sensor in which, in order to reduce the response time, a second pumping cell is provided with which a constant concentration of oxygen in the diffusion channel can be obtained. The second pumping cell serves to achieve the steady state equilibrium condition of the diffusion current in the diffusion channel at an early point in time. Shortening the length of the diffusion channel would also bring about a rapid response time, but at the same time would increase the limit current too strongly. Because of the limited current loading capacity of the electrodes when the concentrations of oxygen are high in a lean gas mixture, a minimum length of the diffusion channel is necessary.
For the use of the limit current sensor in the lean range (.lambda.&gt;1), the stoichiometric range (.lambda.=1) and up to the rich range (.lambda.&lt;1) of the fuel/air ratio, it is known, from EP-B1-190 750, to expose the anode of the pumping cell to a reference atmosphere. In the lean range these sensors operate like the known lean sensors. The oxygen molecules are reduced at the cathode so that the oxygen ions migrate from the cathode to the anode through the solid ZrO.sub.2 electrolyte. At the anode the ions are in turn converted into oxygen molecules and released into the atmosphere. Under stoichiometric conditions, there is a chemical equilibrium at the cathode so that there is no pumping current present. In the rich range, the oxygen ions are also fed from the cathode to the anode as a result of the applied pumping voltage. At the anode, they are in turn converted into oxygen molecules. The stream of oxygen ions flows in the opposite direction from that of the lean range. For this purpose, it is necessary to reverse the polarity of the pumping voltage. This is realized in that the level of the electromotive force occuring under stoichiometric conditions is used as switching signal.
In limit current sensors, a limit current is generally measured with a constant voltage applied to the two electrodes of the limit current sensor. With an oxygen-containing measurement gas, the limit current is linearly dependent on the partial pressure of the oxygen for as long as the diffusion of the gas to the cathode determines the speed of the reaction which is underway. Such limit current sensors which are exposed in particular to the measurement gas are suitable for detecting the concentration of oxygen in lean measurement gases. Between the electrodes, the limit current goes into the lean range as soon as the oxygen molecules passing to the cathode through the diffusion layer are transported away rapidly in the form of ions. In the rich range, the limit current occurs when a diffusion barrier is placed in front of the anode and the diffusion of H.sub.2 and CO to the anode determines the speed of the entire reaction.
When the pumping voltage grows slowly from the 0 volt value, there are ohmic conditions present between the electrodes so that, as the pumping voltage increases, the pumping current rises until the diffusion limit current brings about the limitation of the pumping current. If the cathode did not have a diffusion barrier or if it were exposed to the measurement gas with only a low diffusion resistance, in particular at high partial pressures a diffusion which limits the pumping current would not occur, as a result of which the current/voltage behavior of the sensor would continue to adhere to the ohmic conditions. As a result, the pumping voltage continues to rise so that finally, even at values greater than 1 volt, it does not move into the limit current range and thus it does not become possible to measure the O.sub.2 content. Such high pumping voltages lead to the solid electrolyte and the electrode being destroyed. On the other hand, at low partial pressures, even a low diffusion resistance would be sufficient. However, in order to use the limit current sensor for detecting a wide range extending from lean to rich, a sufficient diffusion resistance must be ensured. A sufficient diffusion resistance which is determined by a corresponding diffusion path of the measurement gas has, in the vicinity of stoichiometric conditions, the disadvantage that there is hardly any difference in concentration any more and thus even small fluctuations of measurement gas falsify the sensor signal. Also, in this ease, even small voltages are sufficient to destroy the solid electrolyte.